1. Field of the Invention
The present invention relates to a rotary dobby used in the art of textile manufacture, and in particular, a rotary dobby of a power weaving loom.
Rotary dobbies provide a means of transmitting movement to the heald frames of a loom, through a rigid linkage, according to a programmed sequence implemented by an electronic device.
2. Description of the Related Art
A variety of prior art dobbies are known in which movement is selectively transmitted to a connecting rod, and hence to the heald frame which is connected to the rod by a linkage.
Certain of these dobbies utilize a key for this purpose. The key is slidably mounted to a cam disk and is received in a socket having a substantially rectangular section shape defined by either the drive shaft or by a drive disk rigidly fixed to the shaft.
This type of coupling constitutes a bilateral locking arrangement, resulting in the creation of a single body between the drive shaft and the cam disk which produces the oscillating movement of the rod.
Other types of dobbies make use of one or more rocking levers mounted to the cam disk, which similarly couple to a complementary socket in either the drive shaft or a drive disk rigidly fixed to the shaft.
All of these prior art dobbies possess one notable limitation, namely the need to suspend the rotation of the drive shaft for some duration.
In effect, a pause in the rotation of the drive shaft is necessary to allow the coupling elements, be they sliding keys or levers, to engage with and disengage from the sockets of the drive shaft or drive disk.
Further, various mechanisms are employed for effecting the engagement of the coupling elements with the drive shaft or disk disk.
These mechanisms range from the more traditional mechanical linkages which, through the use of cams, are actuated by electromagnetic devices in the form of powerful solenoids.
In any event, all of these mechanisms are complex and therefore costly to put into practice.
Similarly, the indexing system by which the drive shaft is made to pause following each rotation of 180.degree., normally consists of a plurality of mechanical elements.
Such elements are generally paired cams, rockers, gears and sectors which must be precision-engineered to guarantee satisfactory levels of performance in operation, and are therefore typified by high costs.
Moreover, the intermittent rotation of the drive shaft, i.e. of the pauses occurring after each half-revolution, result in subsequent high accelerations of the heald frame, which are hardly desirable at the high operating speeds required in particular by jet looms.
Accordingly, and as persons skilled in the art will know, any increase in the frequency of the start-up movement of the heald frames must be accompanied by a reduction in the acceleration of the resulting movement.
Without such a reduction, the system will generate strong moments of inertia, of an order sufficient to weaken the structure of mechanical components and to occasionally cause significant breakages of the warp threads, jeopardizing the output of the loom.
Attempts have been made to overcome the problems associated with the intermittent motion of the drive shaft. These attempts involve releasing the cam disk temporarily from the drive shaft.
Devices featuring such a solution are designed to engage and disengage a cam disk, mounted rotatably in a bearing of the connecting rod operating the heald frame, with and from a drive shaft that rotates continuously.
Accordingly, such devices dispense with the indexing mechanism that would control the movement of an intermittently driven shaft, as well as reduce accelerations in the movement of the heald frames.
With respect to this latter point, those skilled in the art know that the only means of obtaining low acceleration in the movement of the heald frames are governed by kinematic laws of harmonic motion or similar principles.
In the case of a rotary dobby, such movements are obtainable only with the drive shaft rotating at a constant angular velocity.
With this type of solution, centralized selection mechanisms are also rendered unnecessary.
In effect, the movements of the elements serving to connect the cam disk with the drive shaft are induced automatically, simply exploiting the rotation of the main shaft itself.
To this end, the cam disk carries two oscillating levers functioning in the manner of a ratchet.
The two levers in question are arranged in such a way as to engage, in a totally symmetrical manner, two mating surfaces defined by the drive shaft or by a disk rigidly fixed to the drive shaft.
The mating surfaces extend virtually in a radial direction, and are substantially disposed diametrically opposite one another.
The position occupied by the ratchet levers can be advanced relative to that of the mating surfaces defined by the drive shaft.
Following the dead center positions, the levers and mating surfaces engage and the cam disk is caused to rotate.
In this way, for all intents and purposes, there is no need to interrupt the rotation of the drive shaft.
DE-A-2909131 discloses a rotary dobby in which the ratchet levers are operated by a control device that is not described in the documents.
US-A-4625767 and US-A-4730641 disclose rotary dobbies in which the ratchet levers are operated by a plurality of elements such as guides, paired cams, rocker arms and large, powerful solenoids consuming considerable amounts of energy, etc., which must be precision-engineered in order to ensure satisfactory levels of performance. These dobbies are thus expensive to manufacture and put into practice.
Moreover, the overall dimensions of these dobbies prevent them from being capable of being retrofitted to conventional types of power looms in which the space available to accommodate such devices is especially limited.
Also, the basic prior art dobbies comprise elements which cannot withstand any prolonged exposure to the notable stresses generated during the operation of the dobby. As a result, the loom must be stopped frequently to replace such elements, and productivity of the loom is therefore notably reduced.